THE FARM 335 



strongly alkaline, have rapid solvent action upon certain 

 rock materials, accompanied by chemical changes that con- 

 tribute to their reduction. Decaying vegetation in the soil 

 yields acids, and these acting on the rock material aid in 

 bringing about its disintegration. 



The rock known as granite contains among other con- 

 stituents the minerals feldspar and quartz each of which has a 

 definite chemical composition. The feldspar when it is 

 exposed to the action of the atmosphere slowly disintegrates, 

 giving rise to clay. The quartz particles as sand grains may 

 remain mixed with the clay to form loam, or may be separated 

 from it by running water and both deposited in beds sepa- 

 rately as sand and clay. Vast areas of soil often of great 

 depths have been transported by the action of streams and 

 laid down where we find them. In these soils sometimes 

 clay predominates, and sometimes sand. 



SUMMARY 



Rocks are a mixture of various chemical compounds such as quartz, 

 feldspar, hornblende, gypsum, and "limestone". The rock constituents 

 of definite chemical composition are called minerals. 



It may be supposed that originally the earth's land surface was 

 composed of rock only. By reason of expansion and contraction due 

 to temperature changes, by the expansion ol water in the rock crevices 

 when it froze, by the chemical action of carbon dioxide in the air com- 

 bined with water as carbonic acid (H 2 CO 3 ), and by various other 

 agencies, this rock crust became broken and pulverized giving rise to 

 soils. 



So long as it remained in place this soil covering protected the rock 

 below from further weathering. Running water, however, trans- 

 ported this broken rock material elsewhere, and deposited it in enor- 

 mous quantity and to varying depths as sand, clay, and loam. 



Mixed with the earthy material of the soil is more or less of vegetable 

 material in all stages of decay. And in and through it, especially the 

 surface layers, are varied forms of animal and plant life so that instead 

 of being dead inert material the soil commonly may be considered as 

 teeming with life. 



